'Mitochondrial disease' is an enormous collection of conditions, which all have the common feature that the cell mitochondria don't work as they should. Mitochondrial disease itself is a metabolic disorder, so the field of medicine it falls into is metabolic medicine.
Research in the States suggests that 1 in 10 of us is born with some level of mitochondrial dysfunction. Many of us will never know as it will not be severe enough to affect our lives. If you need a lot of sleep to function, your mitochondria might be a bit out of kilter. People live out more or less normal lives with a diagnosis of mitochondrial disease.
It moves along a spectrum from those people to babies who are born so severely affected they die almost immediately.
Each mitochondrial disease has its own features, and adult onset mitochondrial disease is generally less severe but that's not a given.
The depletion syndrome that Charlie has has proved fatal in every other case. Similar syndromes have a different prognosis and take a different course, affecting different parts of the body. People with these conditions are not relevant to Charlie's prognosis.
The rarer the disease, the less certain doctors can be of its course, but the same symptoms will manifest once the mitochondria malfunction, in this case causing severe encephalopathy. So the rarity of the disease doesn't mean the doctors will be unfamiliar with Charlie's condition.
Metabolic disease is an enormously developing field. Links are being made with all sorts of things like ageing, dementia, chronic fatigue; metabolic diseases that can be treated have been added to the heel prick screening and that's made a huge difference to the life chances of those children, who previously could suffer preventable brain damage before diagnoses was made at a later stage.